FIGS. 15A and 15B show a conventional memory device 1500 having an access transistor 1502 and a conductive bridging random access memory (CBRAM) type memory element 1501 (also sometimes called a programmable metallization cell (PMC)). Access transistor 1503 is an n-channel metal-oxide-semiconductor (MOS) type transistor. A memory element 1501 is a two terminal element having a solid electrolyte formed between an anode 1505-0 and a cathode 1505-1.
FIG. 15A shows a conventional programming operation that places element 1501 into a low resistance state. Access transistor 1503 has a source at 0V (which may also be a bit line voltage, VBL), a body at 0V (Vsub=0V), and a gate (which may also be a word line voltage (VWL)) that receives a select voltage Vgs, which is greater than a threshold voltage (Vtn) of the transistor. The anode of element 1501 is biased to a positive program voltage (Vprog). As a result, a program current (Iprog) flows through the element 1501 in the direction from the anode 1505-0 to the cathode 1505-1. Within element 1501, metal atoms from the anode can oxidize and create a conductive path (i.e., filament) through a solid electrolyte layer, thereby lowering a resistance of the element 1501 (with respect to an erased state).
FIG. 15B shows a conventional erase operation that places element 1501 into a high resistance state. Access transistor 1503 has a drain at a positive erase voltage (Verase) (which may also be a bit line voltage, VBL), a body at 0V, and a gate that receives a high power supply voltage VDD. The anode of element 1501 is biased to 0V. As a result, an erase current (lerase) flows through the element 1501 in the direction from the cathode 1505-1 to the anode 1505-0. Within element 1501, metal atoms making up any filament can oxidize, dissolving the filament, thereby increasing the resistance of the element 1501 (with respect to the programmed state).
A drawback to the conventional erase operation of FIG. 15B can be limits in erase current provided to the element 1501. More particularly, in the erase operation shown, access transistor 1503 will introduce a threshold voltage drop (Vtn), reducing the current conduction capability of the transistor as compared to the program operation (shown in FIG. 15A). One way of increasing a maximum current provided by an access transistor 1503 can be to increase the voltage applied at its gate. However, providing such a boosted voltage can require a new power supply source and/or a boosting voltage generator/charge pump(s) and/or cause reliability problems in the access transistor. Further, such an approach adds to the overall erase operation complexity, as an erase operation cannot be controlled only by a transistor source voltage (i.e., the bit line voltage, VBL).
Still further, in some conventional devices, as access transistor sizes are reduced, they may not provide sufficient erase current to erase “strongly programmed” element. Further, erase resistance values can have a wide distribution, and a conventional transistor erase approach (i.e., that shown in FIG. 15B) provides an insufficient range of erase voltages across the element to control erase resistance values.